This invention relates to a method of producing low-voltage aluminum foil capacitor electrodes in which etched and cleaned foil is thermally treated at about 595.degree. to 650.degree. C. and then anodized in an adipate electrolyte which may also contain a phosphate. The process is particularly useful in making low-voltage foil, e.g., less than 100 V and, specifically, 0-60 V foil.
It has been known to thermally treat aluminum electrolytic capacitor foil. Cold rolled, or hard, foil is thermally treated (or annealled) to reduce its brittleness. Aluminum foil has been heat treated to produce a more crystalline oxide, and it has been heat treated during anodization to stabilize it.
Most of these processes are conducted well below the melting point of aluminum except for the annealling process during which there is crystal reorganization and growth.
Anodization of aluminum capacitor foil in both phosphate and dicarboxylic salt electrolytes is well-known also. To produce the barrier layer type anodic oxide, salts of the C.sub.4 -C.sub.10 dicarboxylic acids are generally preferred with the lower acids, specifically adipates, finding more favor because they are more soluble and have good anodization efficiency.
Various electrolyte systems and various heat treatment schemes have been tried to improve capacitance. In some combinations, it was found that heat treatment reduced capacitance of low-voltage foils even though it improved it for intermediate and high-voltage foils.
In copending U.S. application Ser. No. 464,226, filed Feb. 7, 1983 and assigned to the same assignee as the present invention, J. J. Randall, Jr., T. H. Nguyen and C. E. Hutchins described the treatment of etched foil with a phosphate solution followed by heat treatment at 590.degree. to 630.degree. C. to form a thermal oxide layer which modifies the subsequently formed barrier oxide layer. This process described by Randall et al is directed to foils formed at intermediate to high voltages, e.g., 100 V and above. The process of the present invention is directed to improving capacitance at low anodization voltages, e.g., at less than 100 V and specifically 0-60 V.